The invention relates to light amplification and, more particularly, to amplification of an output of a photodiode.
Spectral analysis of living tissue can be used to detect various forms of cancer and other types of diseases. In spectral analysis, light illuminates tissue region under examination and a light detector detects optical properties of the illuminated tissue region by measuring light energy modified by its interaction with the tissue region in a pre-determined frequency and amplitude domain. Optical properties include absorption, luminescence, fluorescence, frequency and time-domain responses to various materials injected to the tissue region and other electromagnetic responses. Diseased tissue may be identified by comparing a spectrum obtained to spectra of normal tissue obtained under the same controlled conditions.
Current devices available for tissue characterization using spectral analysis include night vision sensing systems with filtering adapted to be used with endoscopes and multichannel fiber optic delivery systems. The latter systems typically include a light source, a first optical conduit, a light applicator and receiver, a second optical conduit, a spectrometer and a display unit. The receiver used to receive the reflective light can be a photodiode. A photodiode amplifier can be used to amplify the output from the photodiode.
Photodiode amplifiers are typically used in conjunction with an operational amplifier circuit. An output of a photodiode can be amplified in a photoconductive mode illustrated in FIG. 1. In the photoconductive mode, an amplifier circuit 100 includes a photodiode 110, an operational amplifier (Op-Amp) 115, and a feedback resistor 130. A cathode 110a of the photodiode 110 is coupled to an inverting input 115a of the operational amplifier 115 via a resistor 120. An anode 110b of the photodiode 110 is coupled to a voltage source 140. A non-inverting input 115b of the operational amplifier 115 is coupled to ground 125. An output 115c of the operational amplifier 115 is fed back to the inverting input 115a through the feedback resistor 130. A capacitor 135 is connected in parallel with the feedback resistor 130 to filter out unwanted noise.
Alternatively, an output of the photodiode 110 can be amplified in a photovoltaic mode as illustrated in FIG. 2. The only difference between the photoconductive mode and the photovoltaic mode is the manner in which the photodiode is connected to the operational amplifier. In the photovoltaic mode, the cathode 110axe2x80x2 of the photodiode 110xe2x80x2 is coupled to the inverting input 115axe2x80x2 of the operational amplifier 115xe2x80x2 and the anode 110bxe2x80x2 of the photodiode 110xe2x80x2 is connected to the non-inverting input 115bxe2x80x2 of the operational amplifier 115xe2x80x2.
In both the photoconductive and the photovoltaic modes, a current output from the photodiode 110, 110xe2x80x2 is applied to the operational amplifier 115, 115xe2x80x2. The operational amplifier 115, 115xe2x80x2 provides a voltage output equal to the value of the resistance of the feedback resistor 130, 130xe2x80x2 multiplied by the current from the photodiode 110, 110xe2x80x2 which passes through the feedback resistor 130, 130xe2x80x2. In an xe2x80x9cidealxe2x80x9d situation, the gain of this system is directly related to the feedback resistor 130, 130xe2x80x2 based on the following equation:
VOUT=ILRf
Such an ideal situation is fictional and is rarely realized in practice as other components must be added to compensate for offset currents and voltage drift. These components tend to decrease the gain of the system. Since the gain is decreased and the output is negative, a second amplifier needs to be coupled to the first amplifier in order to provide a positive output voltage with moderate gain. Compensation still takes place with the second amplifier, and this system will still tend to drift slightly. The outcome is a larger system that is not very sensitive and which usually requires re-calibration steps, alignment or other compensatory actions, and is more costly because of the number of components, and the complexity of manufacture.
In one aspect, the invention features an amplifier circuit. The amplifier circuit includes an operational amplifier, a light-sensitive device, a feedback module, and a rectifier. The operational amplifier includes an inverting input, a non-inverting input, and an output. The light-sensitive device is in electrical communication with the non-inverting input of the operational amplifier. The feedback module is in electrical communication with the output of the operational amplifier and the inverting input of the operational amplifier. The rectifier is in electrical communication with the output of the operational amplifier.
In one embodiment according to this aspect of the invention, the rectifier is a rectifying diode. In another embodiment, the light-sensitive device is a photodiode. In another embodiment, the feedback module includes a resistor. In one detailed embodiment, the feedback module further includes a capacitor. In another embodiment, the amplifier circuit further includes a resistance in electrical communication with the inverting input of the operational amplifier. In still another embodiment, the amplifier circuit further includes a resistance in electrical communication with the rectifier.
In another aspect, the invention involves a spectrometer comprising an amplification circuit, a light source for illuminating tissue, and a light-sensitive device. The amplifier circuit includes an operational amplifier, a feedback module, and a rectifier. The operational amplifier includes an inverting input, a non-inverting input, and an output. The feedback module is in electrical communication with the output of the operational amplifier and the inverting input of the operational amplifier. The rectifier is in electrical communication with the output of the operational amplifier. The light-sensitive device is in electrical communication with the non-inverting input of the operational amplifier. The light-sensitive device detects optical properties of the illuminated tissue.
In one embodiment of this aspect of the invention, the light source is internal to the spectrometer. In another embodiment, the light source is external to the spectrometer. In one embodiment, the amplifier circuit comprises a plurality of operational amplifiers. In another embodiment, the spectrometer further comprises a light filter disposed between the illuminated tissue and the light-sensitive device. In one detailed embodiment, the spectrometer further comprises a second light-sensitive device and a second light filter disposed between the illuminated tissue and the light-sensitive device. In another detailed embodiment, the first light filter passes through a first range of wavelengths of light and the second light filter passes through a second wavelengths of light.
In still another aspect, the invention relates to a method for amplifying an output of a photodiode. According to the method, an amplifier circuit comprising a light-sensitive device, a rectifier, and an operational amplifier with an inverting input, a non-inverting input, and a output is provided. A first voltage is applied to the inverting input of the operational amplifier. An optical signal is detected through the light-sensitive device which converts the optical signal to a second voltage. The second voltage is applied to a non-inverting input of the operational amplifier, wherein the first voltage and second voltage have opposite polarity. The output generated by the operational amplifier is transmitted to the rectifier, and an output with the same polarity as the second voltage is transmitted through the rectifier.
In one embodiment according to this aspect of the invention, the method further includes the step of stabilizing the output transmitted through the rectifier. In another embodiment, the first voltage from the output of the operational amplifier is applied to the inverting input of the operational amplifier. In still another embodiment, the method further comprises filtering noise in connection with the optical signal.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.